Meiosis An Introduction An organism must inherit a

Meiosis: An Introduction § An organism must inherit a single copy of every gene from its parents. § Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

Chromosome Number § Different organisms have different numbers of chromosomes. § Humans have 46 chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. § A pair (or set) of chromosmes (one from each parent) are called “homologous”

The “Diploid” Condition § A cell that contains both sets of homologous chromosomes is said to be diploid. § The number of chromosomes in a diploid cell is sometimes represented by the symbol 2 N. § For humans, the diploid number is 46, which can be written as 2 N=46. Diploid

The “Haploid” Condition § Meiosis results in the production of haploid gametes (sex cells) which have only 23 chromosomes. § Meiosis involves two divisions, meiosis I and meiosis II. § By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells (N).

Meiosis I § Interphase I is essentially identical to the process previously outlined in the somatic cell cycle. § Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I

Meiosis I: Prophase I § During Prophase I each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Prophase I § There are 4 chromatids in a tetrad. Tetrad

Meiosis I: Synapsis §Tetrads formed during Prophase I exchange portions of their chromatids in a process called crossing over. §Crossing-over produces new allele combinations and is a key source of variation in multicellular organisms. Tetrad Allele Chromatid

Meiosis I: Metaphase I How is this different from Mitosis? § During Metaphase I, the 23 homologous pairs (tetrads) line up along the equator of the cell. § The assortment of these chromosomes is independent, a primary source of variation within a species.

Alignment at metaphase I or combinations possible Possible Chromosome Combinations or or Independent Assortment at Metaphase I

Meiosis I: Anaphase I § During Anaphase I the homologous pairs separate (via shortening spindle fibers) and move toward the poles of the cell. § Note the difference from mitosis; while the homologous pairs have left one another, the chromatids are still in tact at the centromere. Anaphase I

Meiosis: Telophase I and Cytokinesis § Nuclear membranes form and the spindle breaks down. § Cleavage results in two cells. § The two haploid (N) cells produced by meiosis I have chromosomes from each homologous pair. • The cells are different from each other (remember independent assortment) and from the diploid cell that entered meiosis I.

Meiosis II The 2 haploid cells produced by meiosis I now enter a second meiotic division which results in the production of 4 genetically different haploid cells. No replication takes place. Remember, the chromosomes already replicated during Interphase I.

§Meiosis II Telophase II and Cytokinesis II Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II

Meiosis II § During Prophase II, each of the cell’s 23 chromosomes has 2 chromatids. § Spindle fibers reform and move chromosomes toward cell equator.

Meiosis II • During Metaphase II Chromosomes move to midline of dividing cell, facing opposite poles.

Meiosis II • During Anaphase II, chromatids separate and move toward opposite poles of the cell.

Meiosis II § During Telophase II, the nuclear membrane reforms around chromosomes in each of the four new cells, and a second round of cytokinesis produces four new haploid cells

Sperm or Ovum

Gamete Development… § Spermatogenesis: four new cells from meiosis called spermatids in males develop into mature sperm cells

Gamete Development (cont. ) § Oogenesis: During cytokinesis following meiosis in females, cytoplasm is divided unequally among the four new cells. § One cell receives most of the original cell’s cytoplasm and develops into a mature egg called an ovum. The other three cells called polar bodies die off.

Gamete Development (cont. )

Sexual Reproduction § When the nuclei of the two gametes combine during fertilization, the 2 n number is restored (2 n = 46). § The fusion of two gametes produces the first cell of the new animal called a zygote.

“I want one of my haploid gametes to fertilize one of your haploid gametes…” “OMG! A zygote? . . . How romantic!”

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